Safety is a key issue for batteries, particularly in automobile applications. Organic electrolytes have high vapor pressure, and are often flammable. Molten salt electrolytes have a high melting point and low vapor pressure, compared with organic electrolytes. Therefore, batteries using molten salt electrolytes have the potential for higher safety than those using organic electrolytes. Unfortunately, the molten salt electrolyte can break down in a lithium-based battery, leading to degraded performance. Hence, there is an urgent need for improved lithium-based batteries that can be used reliably in numerous applications.
The lithium sulfur (Li—S) battery has been extensively studied for automobile applications. However, practical use of the Li—S battery is constrained by severe safety problems, and other problems such as self-discharge and low utilization of the sulfur positive electrode. The safety problem is mainly caused by reactions between a low potential Li-metal negative electrode and the flammable organic electrolyte used in a conventional Li—S battery. Furthermore, the organic electrolytes are critical to self-discharge and utilization of sulfur positive electrode. Thus, development of new electrolytes for the Li—S battery is necessary.
The theoretical energy density of a Li—S battery is 2600 Wh/kg, which is much higher than that of a Li-ion battery (510 Wh/kg). Hence, if the safety problem could be overcome, the Li—S battery would find widespread applications in place of the Li-ion battery, and also in place of other batteries failing to provide the high capacity and other advantages of the Li—S battery system. Hence, safety improvements in the field of Li—S batteries have widespread technological significance.
U.S. Pat. No. 5,314,765 to Bates describes a battery having a layer of lithium phosphorus oxynitride coating a lithium negative electrode. However, Bates only describes batteries having organic-based electrolytes, stating that the electrolyte layer can comprise a solid organic polymer containing an inorganic lithium salt, or an organic liquid containing a dissolved lithium salt. No other electrolyte compositions are discussed.